In electrophotographic printers, a print is produced by creating an image of the print on a photoreceptive surface, developing the image and then fusing the image to print material. In machines which utilize plain bond paper or other ordinary image receiving material not specially coated, the electrophotographic process is of the transfer type where a photoreceptive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. In the typical transfer process, photoreceptive material is passed under a stationary charge generating station to place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoreceptive surface. Next, the photoreceptor is moved to an imaging station where it receives light rays which are modulated in accordance with the data to be printed. The light source may be a laser generator, an array of light-emitting diodes, etc., which direct light rays to the photoreceptor and cause it to bear a charge pattern which is a latent image of the information used to modulate the light rays. Modulation is usually derived from a character generator which is driven by image pattern data frequently produced by a computer and held in digitized form in memory.
The digitized image pattern data is organized with an addressability of a specific number of picture elements (pels) per inch and usually with each bit in memory representing a pel; for example, a common database addressability for use with printers is 240 pels per inch. For such a database, the light source is caused to image 240 dots, or pels, on each inch of the photoreceptor image area in both dimensions. Some of these pels represent dark areas, other pels represent background areas, and still other pels represent various intermediate shades. The photoreceptor will be discharged to different voltage levels by the light source to reflect the image pattern data in memory and establish the corresponding pel pattern of dark, background, and intermediate shades on the photoreceptor.
After producing an image on the photoreceptor, the next step in the electrophotographic process is to move the image to a developing station where developing material called toner is placed on the image. This material may be in the form of a colored powder which carries a charge and is electrostatically attracted to those areas which it is desired to develop. Thus, pels representing character printing should receive heavy toner deposits, white background areas should receive none, and gray or otherwise shaded halftone portions should receive intermediate amounts. To aid in attaining these results, a bias voltage is usually placed on the developer station to alter the magnitude of electrostatic fields in the development zone. Thus, the bias voltage is established at a level which provides a field development vector to move the charged toner particles away from the developing station toward the areas to be developed while simultaneously establishing an electrostatic field development vector to move the charged toner particles away from the background areas toward the developing station.
The photoreceptor, now with a developed image, is moved from the developer to a transfer station where a print receiving material, usually paper, is juxtaposed to the developed image. A charge is placed on the backside of the print paper so that when the paper is stripped from the photoreceptor, the toner material is held on the paper and removed from the photoreceptor. Toner remaining on the photoreceptor after transfer is called residual toner and is removed by a cleaning station before the photoreceptor can be reused.
The electrophotographic process is frequently used as a copy process as well as a printing process. In the copy process, a document to be copied is placed on a document glass and light is reflected from the original onto the photoconductor. Since white areas of the original document reflect large amounts of light, the photoreceptive material is discharged in white areas to relatively low levels while the dark areas continue to contain high voltage levels even after exposure. At the developing station, the toner material carries a charge opposite in polarity to the charge pattern on the photoreceptor. Because of the attraction of the oppositely charged toner, it adheres to the surface of the photoreceptor in large amounts on the undischarged areas representing the dark areas of the original document. This process is called a charged area development (CAD) process since heavy toner deposits are made on the heavily-charged areas of the photoconductor after exposure.
In electrophotographic printers, a CAD process can be used, but it has been found preferable to use a discharged area development (DAD) process, primarily because line and character printing results are improved. In the DAD process, the light-generating source, such as the laser or the array of light emitting diodes, etc., discharges the photoconductor in those areas which are desired to be developed; thus, the highly-charged areas of the photoconductor represent white background, whereas the discharged areas represent areas in which toner is to be deposited. In the DAD process, toner material carries a charge of the same polarity as the charge pattern on the photoreceptor. Because of the repulsion of the similarly charged toner, it does not adhere to the highly-charged background areas, but instead deposits in the more lowly charged discharged areas. The DAD process has been successfully used for many years in electrophotographic printers, such as the IBM 3800 printer.
The DAD process, while producing excellent character printing, has (like the CAD process) been characterized by difficulties in printing large solid areas in that a large solid area will frequently be printed with an undesirably dark boundary and an interior that is not dark enough. This represents an excessive deposit of toner on the boundaries of the solid area, and/or not enough toner deposit within the interior of the solid area. Attempts to correct this situation have produced good interior solid areas, but resulted in excessive toner deposition on characters and narrow lines as well as excessive toner deposition at the edges of the solid areas. When excessive toner is deposited on narrow lines, the lines broaden and adjacent lines may become indistinguishable.
While the background of the invention has been provided with reference to electrophotographic printers, the problems of toner balance are found in other non-impact electronic printing processes such as ion deposition and magnetic. The invention herein applies to these other processes as well.
It is an object of this invention to balance the toner deposits made on characters, narrow lines, and large solid areas so that excessive toner deposits do not occur on characters and line drawings, so that excessive toner deposits do not occur along the edges of solid areas and so that toner deposits in the interior of solid areas, are in balance with deposits on characters.
It is another object of this invention to establish control over toner deposition on a pel-by-pel basis. It is still another object to establish such control by differentially exposing each pel in accordance with image pattern data. Finally, it is an object of this invention to provide a procedure for determining the desired levels of differential control to be used in attaining the desired stroke width and toner density on line copy, and the balancing of toner density for solid areas.
The objects of the invention have not been met by the prior art. U.S. Pat. No. 4,517,579 relates to the problem of poorly-developed interiors for solid areas, and teaches a solution of entering white pels among the black ones to create additional fringe fields within the interior of the black area in order to attract more toner. This is obviously a different approach from that taken herein.
U.S. Pat. No. 4,460,909 relates to the smoothing of digitized diagonal lines by producing grey pels at the transition of black and white pels.
U.S. Pat. No. 4,544,264 relates to broadening fine lines, a problem encountered most frequently in CAD process machines.
U.S. Pat. No. 4,491,875 relates to an algorithm for gauging the blackness of a window of pels and then generating a pel pattern to reproduce it.
U.S. Pat. No. 4,595,956 provides an algorithm to recognize transitions from black-to-white and white-to-black.
U.S. Pat. No. 4,463,364 relates to the broadening of fine lines.
U.S. Pat. No. 4,403,257 relates to recognizing the difference between a halftone window and a text window through pattern recognition.
U.S. Pat. No. 4,370,667 relates to obtaining different exposure levels by frequency modulation of pulses provided to drive the light source.
U.S. Pat. No. 4,437,122 utilizes pattern recognition to determine the desirability of altering the pel pattern when converting from one addressability database to another.